There is a clear and immediate change in the pattern of behavior when the researcher switches from a baseline to a treatment phase. This demonstration is necessary to establish a relationship between the treatment and the behavior; that is, a change in behavior accompanies the manipulation of the treatment.
The design includes at least two demonstrations that behavior changes when the treatment is introduced. This replication is necessary to establish a causal relationship between treatment and behavior. It might be argued that the change observed when the treatment is first administered is simply a coincidental effect caused by extraneous variables. However, the fact that the change is replicated when the treatment is administered again at a different time undermines the coincidence argument.